1. Field of the Invention
This invention is related to downhole tools for a hydrocarbon wellbore. More particularly, the invention relates to an apparatus useful in conducting a fracturing or other wellbore treating operation. More particularly still, this invention relates to a collar having valves through which a wellbore treating fluid such as a xe2x80x9cfracxe2x80x9d fluid may be pumped, and a method for using same.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. When the well is drilled to a first designated depth, a first string of casing is run into the wellbore. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. Typically, the well is drilled to a second designated depth after the first string of casing is set in the wellbore. A second string of casing, or liner, is run into the wellbore to the second designated depth. This process may be repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing having an ever-decreasing diameter.
After a well has been drilled, it is desirable to provide a flow path for hydrocarbons from the surrounding formation into the newly formed wellbore. Therefore, after all casing has been set, perforations are shot through the liner string at a depth which equates to the anticipated depth of hydrocarbons. Alternatively, a liner having pre-formed slots may be run into the hole as casing. Alternatively still, a lower portion of the wellbore may remain uncased so that the formation and fluids residing therein remain exposed to the wellbore.
In many instances, either before or after production has begun, it is desirable to inject a treating fluid into the surrounding formation at particular depths. Such a depth is sometimes referred to as xe2x80x9can area of interestxe2x80x9d in a formation. Various treating fluids are known, such as acids, polymers, and fracturing fluids.
In order to treat an area of interest, it is desirable to xe2x80x9cstraddlexe2x80x9d the area of interest within the wellbore. This is typically done by xe2x80x9cpacking offxe2x80x9d the wellbore above and below the area of interest. To accomplish this, a first packer having a packing element is set above the area of interest, and a second packer also having a packing element is set below the area of interest. Treating fluids can then be injected under pressure into the formation between the two set packers.
A variety of pack-off tools are available which include two selectively-settable and spaced-apart packing elements. Several such prior art tools use a piston or pistons movable in response to hydraulic pressure in order to actuate the setting apparatus for the packing elements. However, debris or other material can block or clog the piston apparatus, inhibiting or preventing setting of the packing elements. Such debris can also prevent the un-setting or release of the packing elements. This is particularly true during fracturing operations, or xe2x80x9cfrac jobs,xe2x80x9d which utilize sand or granular aggregate as part of the formation treatment fluid.
In addition, many known prior art pack-off systems require the application of tension and/or compression in order to actuate the packing elements. Such systems cannot be used on coiled tubing.
There is, therefore, a need for an efficient and effective wellbore straddle pack-off system which does not require mechanical pulling and/or pushing in order to actuate the packing elements. Further, is a need for such a system which does not require a piston susceptible to becoming clogged by sand or other debris. Further, there is a need for a pack-off system capable of being operated on coiled tubing.
In the original parent application entitled xe2x80x9cPACK-OFF SYSTEM,xe2x80x9d a straddle pack-off system was disclosed which addresses these shortcomings. U.S. Pat. No. 6,253,856 B1 (the xe2x80x9c856 parent patentxe2x80x9d) is again referred to and incorporated in its entirety herein, by reference. The pack-off systems in the ""856 parent patent have advantageous ability in the context of acidizing or polymer treating operations. However, there is concern that the ports 47 of the pack-off system (such as in FIGS. 1 and 2) may become clogged with sand during a frac job. Therefore, a need further exists for a straddle pack-off system having a specialized collar using larger ports which are opened after the packing elements 40, 41 of the pack-off system have been actuated and set in the wellbore.
Finally, a need exists for a collar within a pack-off system having larger ports to accommodate a greater volume of treating fluid after the packing elements are set.
The present invention discloses a novel collar, and a method for using a fracturing port collar. The fracturing port collar is designed to be used as part of a pack-off system during the treatment of an area of interest within a wellbore. The pack-off system is run into a wellbore on a tubular working string, such as coiled tubing. The pack-off system is designed to sealingly isolate an area of interest within a wellbore. To this end, the pack-off system utilizes an upper and a lower packing element, with at least one port being disposed between the upper and lower packing elements to permit a wellbore treating fluid to be injected therethrough. Exemplary pack-off systems are disclosed in the ""856 parent patent.
The packing elements may be inflatable, they may be mechanically set, or they may be set with the aid of hydraulic pressure. In the arrangements shown in the parent ""856 patent, the packing elements are set through a combination of mechanical and hydraulic pressure. In these arrangements, a flow restriction is provided at the lower end of the pack-off system. A setting fluid, such as water or such as the treating fluid itself, is placed into the pack-off system under pressure. The flow restriction causes a pressure differential to build within the tool, ultimately causing flow through the bottom of the pack-off system to cease, and forcing fluid to flow through the ports intermediate to the upper and lower packing elements. This differential pressure also causes the packing elements themselves to set.
After the packing elements have been set, a treating fluid is injected under pressure through the ports and into the surrounding wellbore. Various treating fluids may be used, including acids, polymers, and fracturing gels. The packing elements are then unset by relieving the applied fluid pressure, such as through use of an unloader. The pack-off system may then be moved to a different depth within the wellbore in order to treat a subsequent zone of interest. Alternatively, the pack-off system may be pulled from the wellbore. To this end, the packing elements are not permanently set within the wellbore, but remain attached to the working string.
The present invention introduces a novel fluid placement port collar into a pack-off system. In accordance with the present invention, the collar is disposed between the upper and lower packing elements. Where a spacer pipe is also used between the packing elements, the collar is preferably placed below the spacer pipe, such as the spacer tube 46 shown in FIG. 1B of the ""856 parent patent.
The collar first comprises an inner mandrel. The mandrel defines an essentially tubular body having a top end and a bottom end within the collar. One or more packer actuation ports are disposed within the pack-off system intermediate the upper and lower packing elements. Preferably, the actuation ports are placed within the mandrel itself intermediate the top and bottom ends. The purpose of the actuation ports is to place the inner bore of the pack-off system in fluid communication with the annular region defined between the outside of the pack-off system and the surrounding casing (or formation).
In the ""856 parent patent, the packer actuation ports are represented by port 47 in FIG. 1B. The actuation ports are of a restricted diameter in order to limit the flow of fluid into the annular region between the pack-off tool and the surrounding formation. This aids in the setting of the packing elements. Setting of the packing elements is accomplished at a first pressure level.
The collar of the present invention further comprises a set of ports disposed in the wall of the tubular mandrel. In one aspect of the present methods, the wall ports define fracturing ports, or xe2x80x9cfrac ports.xe2x80x9d The frac ports are of a larger diameter than the actuation ports in order to permit a greater volume of formation treating fluid to flow through the mandrel and into the formation. In the case of a fracturing operation, the larger frac ports are configured so that they will not become clogged by the aggregate contents of the fracturing fluid. The frac ports are disposed intermediate the top and bottom ends of the inner mandrel, and are placed immediately above or below the actuation ports.
In accordance with the present invention, the frac ports are not exposed to the annulus between the pack-off system and the formation when the packing elements are initially set; instead, they are sealed by a surrounding tubular called a xe2x80x9ccase.xe2x80x9d Once the packing elements are set, fluid continues to be injected into the wellbore until a second greater pressure level is achieved. In this respect, the tubular case of the fluid placement port collar is movable in response to changes in fluid flow rate. In one arrangement, fluid placement port collar is configured so that the case is able to slide axially relative to the outer surface of the inner mandrel. In this respect, the collar is capable of telescopically extending along a designed stroke length. As pressure builds between the packing elements, the packing elements separate in accordance with the stroke length designed within the collar. The frac ports of the collar are ultimately cleared of the case and are exposed to the surrounding perforated casing. Formation fracturing fluid can then be injected into the formation without fear of the ports becoming clogged.